High quality photovoltaic semiconductor material and laser ablation method of fabrication same

Abstract

A high quality, narrow band gap, hydrogenated amorphous germanium or amorphous silicon alloy material characterized by a host matrix in which all hydrogen is incorporated therein in germanium monohydride or silicon monohydride form, respectively; their mobility-lifetime product for non-equilibrium charge carriers is about 10-8 and about 10-7, respectively; their density of defect states in the band gap thereof is less than about 1x1017 and about 2x1016/cm3, respectively; and their band gap is about 1.5 and about 0.9 eV, respectively. There is also disclosed a structure formed from a plurality of very thin layer pairs of hydrogenated amorphous germanium and amorphous silicon alloy material, each layer pair of which cooperates to provide narrow band gap material. From about 3 to about 7 atomic percent fluorine is added to the germanium and/or silicon alloy material so as to provide a strong bond (as compared to hydrogen) so as to provide reduced sensitivity to Stabler/Wronski degradation. The preferred method of fabricating such improved narrow band gap materials is through a laser ablation process in which hydrogen or fluorine gas is introduced for incorporation into the germanium or silicon host matrix, thereby eliminating the reliance on the zoo of precursor species present in r.f. or microwave plasma process.

We claim: 
     
       1. A method of forming hydrogenated amorphous germanium alloy material, said material characterized by (a) the substantial absence of germanium hydrides, other than the monohydride, (b) a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -8  cm 2  /V, and (c) a density of defect states in the band gap of the host matrix thereof of less than about 1×10 17  /cm 3  ; said method comprising the steps of: providing a deposition chamber;   providing a substrate in said deposition chamber;   positioning at least one target of germanium containing material adjacent said substrate;   providing a laser;   evacuating said deposition chamber to a sub-atmospheric pressure;   introducing into said evacuated deposition chamber a background gas including a partial pressure of at least hydrogen; and   depositing, by laser ablation, germanium from said target and hydrogen from said partial pressure onto said substrate; thereby forming on said substrate a layer of hydrogenated germanium alloy material having an amorphous microstructure, said material characterized by the substantial absence of incorporated germanium hydrides, other than the monohydride, a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -8  cm 2  /V, and a density of defect states in the band gap of the host matrix thereof of less than about 1×10 17  /cm 3 .   
     
     
       2. The method of claim 1 including the further step of forming said at least one target of crystalline germanium material. 
     
     
       3. The method of claim 1 including the further step of forming said at least one target of germanium-silicon alloy material or mixtures of germanium and silicon. 
     
     
       4. The method of claim 1 including the further step of forming said at least one target of a germanium material which includes one or more p or n dopant elements added thereto. 
     
     
       5. The method of claim 1 including the further step of forming said layer of hydrogenated germanium alloy material so as to be characterized by reduced porosity as compared with plasma CVD deposited hydrogenated germanium alloy material. 
     
     
       6. The method of claim 1 including the further step of forming said layer of hydrogenated germanium alloy material so as to possess a reduced sensitivity to Stabler/Wronski degradation as compared with plasma CVD deposited hydrogenated germanium alloy material. 
     
     
       7. The method of claim 1 wherein said step of evacuating said deposition chamber to a sub-atmospheric pressure comprises evacuating said deposition chamber to a pressure of about 10 -5  Torr or less. 
     
     
       8. The method of claim 1 wherein said step of introducing said background gas comprises introducing said background gas at a pressure of about 30 to about 1000 m Torr. 
     
     
       9. The method of claim 1 including the further step of incorporating one or more additional components in said background pressure, said additional components being selected from the group consisting of silane, germane, silicon tetrafluoride, germanium tetrafluoride, methane, acetylene, carbon tetrafluoride, fluorine, phosphine, borane, diborane, and mixtures thereof. 
     
     
       10. The method of claim 1 including the further step of heating said substrate before depositing said hydrogenated germanium alloy material thereupon. 
     
     
       11. The method of claim 10 wherein said step of heating said substrate comprises heating said substrate to a temperature of up to 250° C. 
     
     
       12. The method of claim 1 including the further step of laser ablation depositing said hydrogenated germanium alloy material at a laser power of about 3 to about 6 Joules/cm 2 . 
     
     
       13. The method of claim 1 including the further step of laser ablation depositing said hydrogenated germanium alloy material at a laser pulse rate of about 1 to about 50 Hz. 
     
     
       14. The method of claim 1 including the further step of laser ablation depositing said hydrogenated germanium alloy material at a deposition rate of about 1 to about 5 Å/pulse. 
     
     
       15. The method of claim 1 including the further step of controlling laser power and wavelength during said laser ablation deposition process such that free radicals are formed in said deposition chamber. 
     
     
       16. The method of claim 1 including the further step of providing a plasma in said deposition chamber to disassociate gaseous precursors introduced into said background gas into free radical forms thereof. 
     
     
       17. The method of claim 16 including the further step of providing a microwave plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       18. The method of claim 16 including the further step of providing a radio frequency plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       19. The method of claim 1 including the further step of forming said hydrogenated germanium alloy material so as to have an energy gap of about 0.9 eV. 
     
     
       20. A method of forming hydrogenated amorphous silicon alloy material, said material characterized by (a) the substantial absence of silicon hydrides, other than the monohydride, (b) a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -7  cm 2  /V, and (c) a density of defect states in the band gap of the host matrix thereof of less than about 2×10 16  /cm 3  ; said method comprising the steps of: providing a deposition chamber;   providing a substrate in said deposition chamber;   positioning at least one target of silicon containing material adjacent said substrate;   providing a laser;   evacuating said deposition chamber to a sub-atmospheric pressure;   introducing into said evacuated deposition chamber a background gas including at least hydrogen; and   depositing, by laser ablation, silicon from said target and hydrogen from said background pressure onto said substrate; thereby forming on said substrate a layer of hydrogenated silicon alloy material having an amorphous microstructure, said material characterized by the substantial absence of incorporated silicon hydrides, other than the monohydride, a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -7  cm 2  /V, and a density of defect states in the band gap of the host matrix thereof of less than about 2×10 16  /cm 3 .   
     
     
       21. The method of claim 20 including the further step of forming said at least one target of crystalline silicon material. 
     
     
       22. The method of claim 20 including the further step of forming said at least one target of a semiconductor material selected from the group consisting of silicon-carbon or silicon-germanium alloys or mixtures thereof. 
     
     
       23. The method of claim 20 including the further step of forming said at least one target of silicon material which includes one or more p or n dopant elements. 
     
     
       24. The method of claim 20 including the further step of forming said layer of hydrogenated silicon alloy material so as to possess reduced porosity as compared with plasma CVD deposited hydrogenated silicon alloy material. 
     
     
       25. The method of claim 20 including the further step of forming said layer of hydrogenated silicon alloy material so as to have a reduced sensitivity to Stabler/Wronski degradation as compared with plasma CVD deposited hydrogenated silicon alloy material. 
     
     
       26. The method of claim 20 wherein said step of evacuating said deposition chamber to a sub-atmospheric pressure comprises evacuating said deposition chamber to a pressure of about 10 -5  Torr or less. 
     
     
       27. The method of claim 20 wherein said step of introducing said background gas comprises introducing said background gas at a pressure of about 30 to about 1000 m Torr. 
     
     
       28. The method of claim 20 including the further step of including one or more additional components in said background gas, said additional components being selected from the group consisting of silane, germane, methane, acetylene, silicon tetrafluoride, germanium tetrafluoride, carbon tetrafluoride, fluorine, phosphine, borane, diborane, and mixtures thereof. 
     
     
       29. The method of claim 20 including the further step of heating said substrate before depositing said hydrogenated silicon alloy material thereupon. 
     
     
       30. The method of claim 29 wherein said step of heating said substrate comprises heating said substrate to a temperature of up to 250° C. 
     
     
       31. The method of claim 20 including the further step of laser ablation depositing said hydrogenated silicon alloy material at a laser power of about 3 to about 6 Joules/cm 2 . 
     
     
       32. The method of claim 20 including the further step of laser ablation depositing said hydrogenated silicon alloy material at a laser pulse rate of about 1 to about 50 Hz. 
     
     
       33. The method of claim 20 including the further step of laser ablation depositing said hydrogenated silicon alloy material at a deposition rate of about 1 to about 5 Å/pulse. 
     
     
       34. The method of claim 20 including the further step of controlling laser power and wavelength during said laser ablation deposition process such that free radicals are formed in said deposition chamber. 
     
     
       35. The method of claim 20 including the further step of providing a plasma in said deposition chamber to disassociate gaseous precursors introduced into said background gas into free radical forms thereof. 
     
     
       36. The method of claim 35 including the further step of providing a microwave plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       37. The method of claim 35 including the further step of providing a radio frequency plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       38. The method of claim 20 including the further step of forming said layer of hydrogenated silicon alloy material so as to have an energy gap of about 2.1 eV. 
     
     
       39. A method of forming a multi-layered structure of hydrogenated amorphous germanium and amorphous silicon alloy material; the structure defined by a plurality of layer pairs, each germanium and silicon layer of said layer pair characterized by (a) the substantial absence of germanium hydrides and silicon hydrides, other than the monohydrides, respectively, (b) a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -8  and about 10 -7  cm 2  /V, respectively, and (c) a density of defects states in the host matrix thereof of about 1×10 17  /cm 3  and about 2×10 16  /cm 3 , respectively; said method comprising the steps of: providing a deposition chamber;   providing a substrate in said deposition chamber;   positioning at least one target adjacent said substrate, said target including silicon and/or germanium material;   providing a laser;   evacuating said deposition chamber to a sub-atmospheric pressure;   introducing into said evacuated deposition chamber a background gas including a partial pressure of at least hydrogen; and   successively laser ablation depositing silicon and germanium from said at least one target and hydrogen from the background pressure onto said substrate; thereby forming on said substrate a plurality of layer pairs of hydrogenated amorphous germanium and amorphous silicon alloy material, each layer of said layer pair being about 5-30 Å thick, each germanium and silicon layer of said layer of said pair characterized by the substantial absence of germanium hydrides and silicon hydrides, other than the monohydrides, respectively, a mobility-lifetime product for nonequilibrium charge carriers therein of about 10 -8  and 10 -7  cm 2  /V, respectively, and a density of defect states in the host matrix thereof of about 1×10 17  /cm 3  and 2×10 16  /cm 3 , respectively.   
     
     
       40. The method of claim 39 including the further step of forming a portion of said at least one target of crystalline germanium and silicon material. 
     
     
       41. The method of claim 39 including the further step of incorporating one or more p or n dopant elements in the target. 
     
     
       42. The method of claim 39 including the further step of forming each layer of said layer pairs of hydrogenated amorphous germanium alloy material and hydrogenated amorphous silicon alloy material so as to have reduced porosity as compared with plasma CVD deposited hydrogenated germanium or hydrogenated silicon alloy material. 
     
     
       43. The method of claim 39 including the further step of forming each layer of said layer pairs of hydrogenated amorphous germanium alloy material and hydrogenated amorphous silicon alloy material so as to have a reduced sensitivity to photoinduced Stabler/Wronski degradation over time as compared with plasma CVD deposited hydrogenated germanium or hydrogenated silicon alloy material. 
     
     
       44. The method of claim 39 wherein said step of evacuating said deposition chamber to a sub-atmospheric pressure comprises evacuating said deposition chamber to a pressure of about 10 -5  Torr or less. 
     
     
       45. The method of claim 39 wherein said step of introducing said background gas comprises introducing said background gas at a pressure of about 30 to about 1000 m Torr. 
     
     
       46. The method of claim 39 including the further step of including one or more additional components in said background pressure, said additional components being selected from the group consisting of silane, germane, methane, acetylene, silicon tetrafluoride, germanium tetrafluoride, carbon tetrafluoride, flourine, phosphine, borane, diborane, and mixtures thereof. 
     
     
       47. The method of claim 39 including the further step of heating said substrate before depositing said layers of hydrogenated amorphous germanium alloy material and hydrogenated amorphous silicon alloy material. 
     
     
       48. The method of claim 47 wherein said step of heating said substrate comprises heating said substrate to a temperature of up to 250° C. 
     
     
       49. The method of claim 39 including the further step of laser ablation depositing each of said layers of said layer pairs of hydrogenated amorphous germanium alloy material and hydrogenated amorphous silicon alloy material at a laser power of about 3 to about 6 Joules/cm 2 . 
     
     
       50. The method of claim 39 including the further step of laser ablation depositing each of said layer pairs at a laser pulse rate of about 1 to about 50 Hz. 
     
     
       51. The method of claim 39 including the further step of laser ablation depositing each of said layer pairs at a deposition rate of about 1 to about 5 Å/pulse. 
     
     
       52. The method of claim 39 including the further step of controlling laser power and wavelength during said laser ablation deposition process such that free radicals are formed in said deposition chamber. 
     
     
       53. The method of claim 39 including the further step of providing a plasma in said deposition chamber to dissociate gaseous precursors introduced into said background gas into free radical forms thereof. 
     
     
       54. The method of claim 53 including the further step of providing a microwave plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       55. The method of claim 53 including the further step of providing a radio frequency plasma in said deposition chamber during said laser ablation depositing step. 
     
     
       56. The method of claim 39 including the further step of introducing a partial pressure of fluorine-containing gas into the deposition chamber, whereby each layer of the layer pairs will incorporate into the host matrix thereof a substantial atomic percentage of fluorine so as to create more stable bonding configurations which are less sensitive to photoinduced degradation. 
     
     
       57. The method of claim 56 wherein about 3 to about 7 atomic percent of fluorine is incorporated into each layer of the layer pairs. 
     
     
       58. The method of claim 39 wherein each interface formed between each germanium layer of each layer pair and each silicon layer of each layer pair is discrete. 
     
     
       59. The method of claim 39 wherein two targets are provided, one of said targets including germanium material and the second of said targets including silicon material. 
     
     
       60. The method of claim 39 wherein a single target is provided and the target includes regions of germanium material and regions of silicon material.